Alumina (Al2O3) for production of aluminum is largely produced from bauxite (more than 95 wt %). However, in recent years, the availability of good grade bauxite has diminished and the price has correspondingly increased. The bauxite processing generates environmental problems (e.g. red mud) particularly when processing lower grade bauxite. For these reasons, considerable interest has been put on production of alumina from aluminum rich silicate rocks, such as anorthosites, nepheline syenites and feldspar/feldspathoid minerals derived from such rocks, as it is known that these rocks and minerals can be dissolved directly in strong mineral acids, without any costly pretreatment step, such as high temperature roasting.
Particularly anorthosites, with high anorthite content have received much attention, an example being Norwegian patent No. 323417 (Eriksen et al.). Lately sedimentary rocks, such as argillite (clay/mudstone) have also received considerable interest. ‘Anorthosite’ is a collective term for igneous rocks characterized by a predominance of plagioclase feldspar (90-100%), and a minimal mafic component (0-10%). The plagioclase feldspar series contains a variety of Na—Ca—Al silicates between the two end members albite (NaAlSi3O8) and anorthite (CaAl2Si2Og). Norway has abundant occurrences of anorthosite, some with high anorthite content (70 to 80%) located at the western coast. Due to the high alumina content (Al2O3>30%) in the Gudvangen deposit (estimated at >500 M Tonnes of anorthosite) located in Sogn og Fjordane, recovery of alumina from Norwegian anorthosite has been subject to extensive studies.
One of the largest research efforts was invested into the Anortal project (1976-1987), a process to produce alumina from anorthosite, based on leaching or dissolving the mineral with a mineral acid and the subsequent precipitation of aluminium trichloride hexahydrate (AlCl36H2O) from the acid phase. A technological path for a nitric acid route was patented (U.S. Pat. No. 4,110,399 A) by the Institutt for Atomenergi (now IFE). The technological concept was later developed and patented by Eriksen et al. as Norwegian patent No. 323417. The process according to this patent relies on leaching with nitric acid followed by subsequent solvent extraction of unwanted species (Fe, Ca) and partial acid recovery. Worldwide, other attempts have been made to obtain alumina by alternative process different from Bayern:
U.S. Pat. No. 4,110,399 (Gaudernack et al., 1978) shows a process for extraction of alumina from Al containing silicates involving leaching with sulphuric acid, extraction of iron into an organic phase while leaving Al ions in the aqueous phase, precipitation of Al as aluminium chloride hexahydrate and subsequent calcining.
U.S. Pat. No. 4,367,215 claims the production of silica with controlled properties by acid leaching of silicates, but limits the scope to the silica product and with no technological solutions for alumina or carbonates production, acid recovery, iron separation, etc.
CA patent No. 2,711,013 Al proposes an invention for the obtaining of Al from aluminous ores, by initial dissolution of the ore with acid, but focusing on the later separation of aluminium and iron ions to produce iron-rich concentrate and the later extraction of aluminium by organic extraction. Therefore, neither a sparging step for the initial aluminium separation, nor the CO2 use for carbonates precipitation and nor the acid recovery by amines thermal treatment are considered in that process.
U.S. Patent application No. 2009/022640A1 proposes a process where sulfuric acid is used for leaching the aluminium-containing solid and the later use of hydrochloric acid during the sparging step is done at a temperature under 20° C. U.S. patent application No. 2012/0237418 A1 (Boudreault, Alex and Biasotto) describes a process to obtain aluminium by leaching with hydrochloric acid (the pressure is not specified) and the later separation of iron from aluminium by several pH-controlled stages by using organic extractants, therefore focused in high iron content aluminium ores (e.g. argi!!ite, nepheline). The aluminium and iron separation follow different methods and the use of CO2 and carbonate production nor the acid regeneration are mentioned.
U.S. Pat. No. 4,158,042 proposes the dissolution of the Al-rich mineral with a leaching liqueur containing chloride, calcium and fluoride ions, this last used as reaction catalyst (in the form of H2SiF6 and in a quantity of 1-20 gms/liter). When applied to a Ca-rich rock (anorthosite), they propose the precipitation and separation of part of the CaCl2 and the combination of this CaCl2 with silica at high temperature (1100° C.) to recover part of the HCl. This sub-process for acid recovery is very energy demanding, with a highly negative impact on the possible profitability of the process.
For the separation of Al from the leaching liqueur, John E. Deutchman and Francoise Tahiani (U.S. Pat. No. 4,472,361, 1984) reported a method to separate Al and Na from a starting solid mixture of AlCl3 and NaCl (coming from a quantitative precipitation by a first sparging) applying a selective redissolution of the AlCl3 in water, to produce an aqueous AlCl3 solution with a reduced Na concentration, and a solid NaCl product that can be separated by filtration. A second sparging with HCl gas is used to re-precipitate AlCl3 from the aqueous solution. After separation of the AlCl3 (i.e. ACH), the concentrated HCl solution is recirculated to the process in the first sparging step, while the solid ACH is sent to the calcination process step.
For the separation of iron, in U.S. Pat. No. 5,585,080, a method for recovering metal chlorides from silicon and ferrosilicon is described. In that work, TBP was applied for iron chloride extraction, directly after the leaching of the material, from the acid solution containing high AlCl3 and CaCl2concentrations, followed by sparging of HCl gas to recover aluminium chloride. After removal of FeCl3, the leachate consists of a concentrated HCl solution with metal chlorides such as CaCl3, MgCl2, NaCl.
Regarding the recovery of the process acid, several patents present the possibility of using organic extraction (with different amines) to extract free HCl from diluted solutions and for the later recovery of concentrated HCl by stripping of the amine (Baniel and Jansen, U.S. patent application No. 2012/0134912; Baniel and Eyal, U.S. patent application No. 2010/0093995, U.S. patent application No. 2011/0028710 and EP 2 321 218 Al; Baniel, Eyal and Jansen, WO 2010/064229 A2; Coenen, Kosswig, Hentschel and Ziebarth, U.S. Pat. No. 4,230,681; Willi Ziegenbein, Ferdinand von Praun, U.S. Pat. No. 4,272,502 A; DeVries, U.S. Pat. No. 4,640,831). These publications are applicable for the recovery of free HCl in solution, but not for recovering Cl″ ions from metal chlorides with precipitation of the corresponding metal carbonate. Other authors have proposed the CO2 utilization for the precipitation of sodium bicarbonate (Hentschel, Coenen, Kosswig, von Praun and Ziebarth U.S. Pat. No. 4,337,234; Coenen, Laach, Kosswig, von Praun and Hans Regner U.S. Pat. No. 4,321, 247 A; Hentschel, Jurgen, Coenen, Kosswig, Ferdinand von Praun U.S. Pat. No. 4,320,106A), and for the production of ammonia from ammonium chloride (Coenen, Laach, Kosswig Dieter U.S. Pat. No. 4,305,917), but not tackling the later acid recovery from the amine.
The most recent patent application related to alumina production—WO 2013/037054 Al is based on the well-known generation of dissolved metal chlorides by the leaching of an aluminium-rich material with HCl, and the later re-precipitation of the metal chlorides by sparging with HCl. Then, the acid recovery is achieved only by the calcination of the diverse metal chlorides obtained along the process (AlCl3-6H2O, FeCl3-xH2O, MgCl2-xH2O, etc.) to evolve the HCl as a gas and produce metal oxides. However, low total HCl recovery can be expected if this process is applied to any Al-containing materials that have a high Ca-content since the hydro-pyrolysis of CaCl2 is difficult due to its low melting point and the high decomposition temperature of CaCl2-2H2O. Additionally, no technological solution is given for the efficient separation of sodium if the ore contains this element, which would precipitate as NaCl together with the aluminium chloride during the sparging step. This means that applying this method to e.g. anorthosite, would be doubtfully economic, due to its considerable calcium and sodium content. So technically, several Al-rich could be treated following WO 2013/037054 Al steps, but obviously only some minerals—especially those rich in iron and magnesium—are the most appropriate raw materials that could bring a competitive process. As in the previous alumina production patented alternatives, the use of CO2 and the recovery of extra HCl while producing carbonates from the remaining chlorides in solution is not mentioned.
Therefore, although some of the alternative process concepts succeeded with respect to product recovery, either the economic viability of those technologies proved unfavorable in comparison to the already well-established bauxite Bayer process and/or focused on only parts of the process or did not tackle acid recovery as to make it applicable for varied aluminium sources.